I don't need an amp for my ATH-AD700s either, they are famously easy to drive and on paper an extra amp is too much for it. But sometimes the extra punch of the gain 6 or even gain 12 setting on the E17 is nice, certain songs or movies. I always try max volume on 0 gain first because the less gain the less distortion apparently, but with some sources sometimes (not often) the extra power the E17 offers is nice.
AD700 is an excellent headphone btw. I spent almost four times as much on top rated powered speakers (Aktimate mini+) and the speakers don't sound anywhere near as good.